Simon toy that's integrated into an ear training tool?

Blindfold chess with Last N moves hidden?

Mensa-style slide puzzles?

An extension that converts random words into phonetic equivalents like morse, braille, and vorticon?

I've also made some way less useful stuff like a win32 app that lets you physically grab a window and hurl it which invokes an WM_DESTROY when it completely is off the screen.

And an app that measures low frequencies to tell if you are blowing into the mic and then increases the speed of the CPU fan to cool it down.